An experiment was conducted over three consecutive seasons (2009, 2010, and 2011) at three locations , Rahad Research farm. Gedarif Research Station farm (North Gedarif and South Gedarif region) of the Agricultural Research Corporation (ARC), Sudan. Both North and South Gedarif were rainfed, while Rahad station was irrigated. A randomized complete block design with four replicates was used. Sorghum production is highly influenced by the environment where it is grown, thus, the genotype by environment interaction is highly significant when breeding for specific adaptation. The objective was to assess the genotype x environment interaction and stability of grain yield. The mean squares due to environment, genotypes and genotype x environment interaction were highly significant for grain yield. Significant differences among genotypes for the studied characters were found in almost all seasons, indicating that these sorghum genotypes were highly variable for the characters studied and , therefore, expected to respond to selection. The interaction effects of genotype x location were highly significant for most traits indicating that genotypes responded differently to different environments and some are environmentally specific. The present study showed that the first two axes PCA1,PCA2 in Additive Main Effect and Multiplicative Interaction (AMMI ) accounted for the GE sum of squares by 56.7% and 19.3%, respectively, while the regression analysis accounted for GE sum of squares by 21.9% .Hence, AMMI analysis was superior to regression techniques and more effective in partitioning the interaction sum of squares. From both statistical stability models used in this study, i.e. Eberhart and Russell (1966) as well as the Aditive Main Effect and Multiplicative Interaction (AMMI) analysis, they pointed out that genotypes Mugod (1510 kg/ha), Tabat (1299 kg/ha), Wad-Ahmed (1471 kg/ha), Gadambalia bloom (1428 kg/ha), Safra (1410 kg/ha) and Tetron (1323) were high yielding and stable under the favorable environments of South Gedarif and Rahad irrigated Scheme. Genotypes Wad Baku(1225 kg/ha), Farhoda (1252 kg/ha),Gesheish (1194 kg/ha) and Wad Fahal (1230 kg/ha) were low yielders but quite stable under low rainfall environments like North Gedarif environment.

INTRODUCTION

Sorghum (Sorghum bicolor (L) Moench) is an important food and feed crop. As an energy supplier for the world’s population, it ranks sixth, and it is fifth in importance among cereals. Semi-arid tropical Asia and semi-arid tropical Sub-Saharan Africa grow about 60% of the world area (ICRISAT and FAO, 1996), while Sudan grows about 24% of Africa area and produces 17% of its production. The national average yield in the Sudan (250 kg/fed) was 18% of that obtained at the research stations (Ishag and Ageeb 1987). This was attributed among many other factors, to the use of low yielding cultivars as well as to poor cultural practices.

During the last 15 years, plant breeders in the Agricultural Research Corporation (ARC) have successfully developed high yielding open pollinated varieties such as Feterita, Wad Ahmed, Ingaz (Osman and Mahmoud,1992) and Tabat (Osman et al.1996). In addition, many other varieties suitable for both irrigated and rainfed sectors were also developed such as Butana and Bashayer (Elzein et al.,2008), and AG-8 (Abdalla et al., 2009).

Estimation of stability performance has become an important tool to identify consistently high-yielding genotypes (Kang,1998). Many stability statistical methods have been used to determine whether or not cultivars evaluated in multi-environment trials were stable (Lin et al.,1986; Flores et al.,1998; Hussein et al.,2000; Robert,2002).The use of a method that integrated yield performance and stability for superior genotypes becomes important because the most stable genotypes were not often the highest yielding (Kang and Magari ,1996).

Conventional methods of partitioning total variation into components due to variety, environment and variety-environment interaction conveyed little information on individual patterns of response (Kempton ,1984). Other methods used include regression analysis to partition genotype x environment interaction (Gauch,1988), and multivariate analysis (Westcoff,1987). Development of sorghum with high yielding and desirable grain quality for different environments is one of the exciting research that leads to successful evaluation of stable genotypes which could be used for general cultivation or as breeding material. Therefore, the objective of this study was to assess genotype x environment interaction and stability of sorghum grain yield using regression method of Eberhart and Russels, (1966). The deviation from regression is used to assess unpredictable part of variability

of any genotype with respect to environment that could not be predicted by the regression. It is a measure of reliability of the linear regression and the stable genotype was defined as one with bi = 1, S2d = 0 and higher than the overall mean grain yield , and more recent application methods such as Additive Main and Multiplicative Interaction analysis (AMMI). Multivariate analysis such as AMMI analysis groups genotype or environments in a qualitative manner according to their similarity of performance rather than quantitative manner of the stability parameters. AMMI analysis involves the clustering analysis to classify genotypes under the most adapted sites for them depending on the AMMI principle components scores (Gauch and Zobel,1988; Nachit et al. 1992). Non parametric approach (multivariate) has been proposed to overcome problems associated with parametric approach (Lin et al,1986).

MATERIALS AND METHODS

Location

The experiments were conducted over three consecutive seasons (2009, 2010 and 2011) at three locations,viz. Rahad Research Farm , North and South Gedarif regions of the Gedarif Research Station farm of the Agricultural Research Corporation (ARC), Sudan . The three locations lied within the central clay plain of the Sudan, characterized by heavy alkaline clay soil, with a pH of around 8.5 and low in nitrogen and organic matter.

Plant material

Eighteen accessions of sorghum collected from Gedarif and from the gene bank (Wad Medani) were used in this study. These accessions were five released varieties (Wad-Ahmed, Tabat, Butana, Bashayer and Arffagadamak-8), and 13 local land races preferred by farmers (Korakollo, Mugod, Saffra, Wad-Bako, Tetron, Faki-Mustahi, Farhoda, Gadambalia bloom, Ajeb-seido, Arafah, Gesheish,Wad-fahal and Milo) .

Cultural practices

The standard cultural practices adopted for sorghum at the ARC were followed. Land was prepared by disc ploughing, disc- harrowing, leveling and ridging in irrigated site and by disc- harrowing in rain-fed sites. Treatments were laid out in a randomized complete block design with four replicates in the different locations and seasons. Sowing was done in the

first week of July under irrigation and the first to the third week of July under rainfed conditions depending on the onset of rainfall. Under irrigation, the entries were sown in five rows, 5 m long on ridges; 0.8 m apart at 0.3 m intra - row spacing and thinned to two seedlings per hill. Under rainfed conditions, they were also sown in five rows 5 m long on flat; 0.8 m apart at 0.2 m intra row spacing and thinned to two seedlings per hill. Urea at the rates of 80 kg and 40 kg /fed was applied under irrigation and rainfed sites, respectively, as recommended by the ARC. The crop was irrigated every two weeks or whenever necessary and irrigation was withheld three weeks before harvest. In irrigated and rainfed experiments, assessments were made in the central three rows of the plot discarding one row or more at each side. Data were collected on days to 50% flowering, plant height, number of heads/m², head length (cm), head width (cm),1000 seed weight(g) and grain yield (kg/ha).

Statistical analysis

The analysis of variance procedure was used to test differences among genotypes within each season, location and combined. Eberhart and Russell (1966) stability model was performed. In addition, the Additive Main Effect and Multiplicative Interaction (AMMI) was carried out to show the stability and pattern of adaptation of sorghum genotypes in nine environments, using IRRISTAT(2005) statistical analysis package for grain yield data.

RESULTS AND DISCUSSION

The combined analysis of variance showed highly significant differences among seasons for all the traits studied with the exception of head length (Table 1).It also showed that differences among locations were highly significant for all traits under study. Differences among genotypes were highly significant for all traits with the exception of number of plants/m² and head length. The interaction effect of genotype x location was highly significant for most traits except number of plants /m² and number of heads /m² and this may be due to genetic factors.

The significance of genotype x environment indicated that genotypes responded differently to environments and some are environmentally specific. Also, this finding indicated the importance of these components in affecting the phenotypic performance of the evaluated genotypes in the different environments. Similar results were reported by Abdalla et al.

(2009), who found that the mean squares of genotypes, environments and genotypes x environments interactions are highly significant (P=0.01). Also Elasha et al.(2011) found significant differences between environments, genotypes and environment x genotype.

Genotypes significantly interacted with seasons for almost all traits except number of plants/m² and head width and this may be due to genetic factors. However the significant interactions of genotypes with seasons shown by all of the characters studied reflect their instability over seasons. Similar results were reported by Shivanna et al.(1992) and Santos et al. (1995). The second degree interaction of season x location x genotype was significant for all traits except for number of plants/m². Kambal and Mahmoud (1978) reported that variety x year interaction was small and not significant, while the variety x location and variety x location x year interaction were highly significant in sorghum.

The current findings indicated that there is a wide range of genetic variability among tested genotypes, which could be attributed to both genetic and environmental factors and their interactions. Similar results were reported by Hashim (2008) and Bello et al (2007) who studied genetic variability in sorghum and reported significant differences among cultivars for days to 50% flowering, plant height, 1000- seed weight and grain yield .

Shinde and Jagadshwar (1986) in F1 and F2 generations of 8x8 diallel cross evaluated for grain yield and yield components in three environments showed significant genotype x environment interaction for all studied traits. From the present study, and on the basis of the importance of genotype x environment interactions as shown it could be concluded that sorghum genotypes show differential responses when grown under different environments, suggesting that these genotypes should be tested in different environments.

S X LXG

S X G

L X G

Genotype (G)

Location (L)

Season (S)

Trait

141**

245*

126**

1274**

7091**

2068**

DF

1775**

2333**

2057**

1323**

246696**

19500**

PH

7.93 ns

9.11ns

9.60 ns

7.84 ns

909**

55**

P/m

10.30*

13.3**

7.67 ns

28.7**

4096**

83.9**

H/m

22.83**

56.23**

18.84**

29834 ns

15.5**

573 ns

HL

1.5**

1.13 ns

1.8**

2.5**

75.2**

74.8**

HW

46**

55**

113**

423**

7251**

660**

Sw/

143870**

165846**

165846**

166993**

7349446**

24873664**

GY

Table 1. Means for seasons , locations ,genotypes and their interactions for18 sorghum genotypes combined over three seasons and three locations, grown at North Gedarif, South Gedarif, and Rahad Research farm (RRF) during season 2009,2010,and 2011.

The data on the three stability parameters, mean performance, regression coefficient(bi) and deviation from regression (S²d) for grain yield are presented according to Eberhart and Russell (1966) stability model (Table 2). The mean grain yields of sorghum genotypes ranged from 846 kg/ha as minimum to the 1510 kg/ha as maximum , with an average of 1302 kg/ha. Seven genotypes recorded higher yield than the mean of all genotypes (Table 2).These genotypes were Tetron (1323 kg/ha), Butana (1333 kg/ha), Safra (1401 kg/ha), Gadambalia bloom (1428 kg/ha), Wad-Ahmed (1471kg/ha), Bashaiyer (1503 kg/ha), and Mugod (1510 kg/ha).

Genotypes with bi > 1 and mean grain yield greater than the general mean, were Mugod, Safra, Tetron, W-Ahmed and Gadambalia bloom indicating that they were more responsive to environmental changes and, therefore, suitable for favorable environments of irrigation conditions (Rahad) and high rainfall conditions (South Gedarif).

These findings agreed with those reported by Elasha et al. (2011) who studied stability and adaptability of seven hybrids and three open pollinated varieties under twelve environments. They found that the genotypes DIA-07666, DMN 15P 1003, PAC-501 and E-1 showed slopes (bi) of 2.67, 2.49, 2.34 and 1.18 with deviation from regression of 0.08, 0.45, 0.68 and 0.12 under irrigation, respectively, and a mean grain yield above the general mean of the traits meaning those are more adaptable under irrigation conditions.

Genotypes with (bi) close to 1.0 but low yielding (below the general mean),and so quite stable with relatively small S2d were Korakollo,Wad Baku, Farhoda, Gesheish, and Wad Fahal (1272, 1225, 1252,1194 and 1230kg/ha, respectively).This means that these genotypes have better response under unfavorable environments and are, therefore, stable and adaptable. Similar findings were reported by Abdalla et al.(2009) who studied stability and adaptability in some sorghum lines grown under nine environments . They found that genotypes, AG15 and AG8 had b values of 0.900 and 0.928, repectively,The Genotypes, AG15 and AG-8 also had mean grain yield of 894.35 and 862.32 kg/fed respectively, and Wad Ahmed had 862 kg/fed which were above the overall mean of 777.73 kg/fed of the trials, while CAG had a mean of 543kg/fed, which was lower than the overall mean. This means that both genotypes have better response in unfavorable environments and are, therefore, adaptable stable and predictable (high R2 value) than the two checks. Similar results were also reported by Elzein et al. (2008) who studied stability and adaptability in some sorghum lines and they found regression coefficients greater than one and had higher(S2d) observed for Gew 22-15 and Gew 3-2 with mean grain yield below the general mean yield, indicating that these two lines were not stable under adverse conditions but may respond better to favorable environments.

The most stable genotypes as indicated by this stability parameter were Mugod, Tabat, Gadambalia bloom, Safra, Wad Ahmed and Tetron when the mean yield, regression coefficient and the deviation from regression were considered together.

In the present study, multivariate analysis such as AMMI analysis groups genotype or environments in a qualitative manner according to their similarity of performance rather than quantitative manner of the stability parameters. AMMI analysis involves the clustering analysis to classify genotypes under the most adapted sites for them depending on the AMMI principle components scores (Gauch and Zobel,1988;Nachit et al. 1992). The combined analysis of variance according to the AMMI model is presented in Table 3.

The partitioning of GE interaction through AMMI model analysis revealed that the four multiplicative terms (PCA1, PCA2, PCA3, and PCA4) were significant and were captured 56.7%, 19.3%, 10.1%, and 7.2% of variation due to GE interaction sum of squares, respectively. Together they accounted for 93.3% of GE interaction sum of squares. However, most of the variation was explained by the first principle components (PCA1).

According to Crossa et al. (1990), AMMI with two, three or four PCA1 axes is the best predictive model. Similarly, in the present study, the AMMI analysis further revealed that the first two interaction principle component axes (PCA1 and PCA2) explained 76% of the GxE sum of squares. This was in agreement with Sneller et al., (1997),who suggested that GxE pattern is collected in the first principal components of analysis.

Table 3. AMMI analysis of variance of the significant effects of genotypes (G), and environment (E) and genotype- environment interaction (GE) on grain yield (kg/ha) and the partitioning of the GE into AMMI scores.

Source of variation

DF

SS

MS

Efficiency (%)

Environment (E)

Genotypes (G)

GE I

PCA1

PCA2

PCA3

PCA4

Residual

8

17

136

24

22

20

18

52

0.25367E+0.8

706700

0.604554E +0.7

0.342852E +0.7

0.116852 E +0.7

615978

392181

440344

0.317095E+0.7

41570.6

44452.5

142855***

53114.5***

30798.9**

21787.8**

100

56.7

19.3

10.1

07.2

**,*** Significant at the 0.01 and 0.001 probability levels, respectively.

Variation among the studied genotypes for grain yield and their reactions to the environments were determined (Table 4). The highest average yield was obtained in E-7 followed by the E-9 (representing Rahad environment), whereas E-1 (representing North Gedarif environment) had obtained the lowest grain yield. E-7 exhibited the largest absolute PCA1 score (i.e. had the highest interaction effect), whereas the smallest score was shown by the E-4 ( representing South Gedarif environment) (i.e. had the least interaction effects). Based on AMMI biplot, G and E having PCA values close to zero have small interaction effects, whereas those having large positive or negative PCA absolute values largely contribute to GE interaction. Hence, E-7 was the most interactive, while E-4 was the least interactive among the nine environments.

Table 4. PCA1 and PCA2 scores for the nine growing environments

of sorghum genotypes.

Environment

E-Mean

IPCAe (1)

IPCAe (2)

E1

113.8

3.51928

6.33557

E2

144.3

3.39088

6.43972

E3

310.3

5.28205

-4.16294

E4

631.2

-3.50394

4.74171

E5

671.1

5.81292

17.75241

E6

475.3

7.83628

-1.66849

E7

1215.9

-39.053

-5.03838

E8

191.4

4.84558

0.39725

E9

1184.2

11.86991

-24.7968

E1,E2,E3 (North Gedarif ),E4,E5,E6 (South Gedarif),E7,E8,E9 (Rahad).

To analyze genotype-environment interaction and adaptation graphically, AMMI biplot was used with the PCA score plotted against the mean yields (main effects).

A graphical display of the GE interaction of PCA1 and their effects (yields) is useful for revealing favorable pattern in genotypes response across environments (Crossa et al.1990). The AMMI bi-plot of mean on yield explained a large proportion of the treatment sum of squares. The PCA scores, negative or positive, more specific or adaptive genotype to certain environments. The more PCA score approximate to zero, the more stable or adapted genotype over all environments. Accordingly, the genotypes Mugod, Safra ,Tetron, Gadambalia bloom, Butana ,and Bashaiyer revealed good stability across environments and high grain yields. This indicated that these genotypes Butana,and Bashaiyer were stable over all environments, while the genotypes W-Ahmed,Tabat, Mugod, and Safra were adapted for specific environments. W-Ahmed and Tabat for favorable environment, while Mugod and Safra were adapted for specific environment (South Gedarif environments). Genotype Mugod exhibited high yield in the environment 6 which represent South Gedarif environment,followed by the genotypes Gadambalia bloom,Safra, and genotype Tetron, respectively. (Fig 1).

To further explain the GE and stability,a bi-plot between the PCA1 and PCA2 scores were given in Fig2. AMMI bi-plot of the first two principle component axes is a powerful way of detecting important score of GE effects (Zobel et al.1988).This analysis represents stability of the genotypes across environments in terms of principle component analysis. It is used to identify broadly adapted genotypes that offer stable performance across sites, as well as genotypes that perform well under specific conditions. In this study, the first two principal component axes (PCA1 and PCA2) in bi-plot analysis explained a large proportion of the variation 76% of the total GE sum of squares (Table 3).On this AMMI bi-plot ,genotypes and environment ِhaving PCA values close to zero (near the origin) have small interaction effects, whereas those having large positive or negative PCA values (distant from zero) largely contribute to GE interaction (Yau,1995). Hence, the genotypes Butana, Farhoda, Faki-Mustahi, ashaiyer, Gadambalia bloom, Safra, and Wad baku were the most interactive ,while the genotypes W-Ahmed, Tabat, Wad Fahal,and Gesheish were the least interactive. On the other hand, environments E-9 and E-6 appeared we distant from the origin (large PCA score), hence they had large interaction effects, whereas E-2 had small interaction effects (Fig.2). Genotypes Tabat,W-Ahmed, and Wad Fahal were more stable and responsive for good environments (Rahad environment),while the genotypes Mugod, Tabat ,Wad-Ahmed, Safra, and Tetron were responsive and suitable for South Gedarif environment. Hence, in this investigation, visual observations of AMMI bi-plot analysis enable the identification of genotypes and testing environments that exhibited major sources of GE interaction as well as those that were stable. Similar results were reported by Sneller et al (1997). From the result shown in Table 4 and Fig 2, it was found that the genotypes Mugod, Wad-Ahmed, Tabat, Gadambalia bloom, Safra and Tetron were high yielding and stable under favorable environments, and they could be grown under high rainfall and Rahad conditions. The others (Wad Baku, Farhoda, Gesheish and Wad Fahal) were quite stable under unfavorable conditions, and it could be grown under low rainfall conditions of North Gedarif. In this study, comparing the effectiveness of joint regression and AMMI analysis for analyzing GE interaction, it was found that PCA1 in AMMI accounted for the GE sum of squares by 56.7%, while regression analysis accounted forGE sum of squares by 21.9%. Hence, AMMI analysis was superior to regression techniques in accounting for GE sum of squares and more effective in partitioning the interaction sum of squares.

From these two models of stability used in this study, it was found that the genotypes Mugod, Wad-Ahmed, Tabat, Gadambalia bloom, Safra and Tetron were high yielding and stable under favorable environment, and could be grown under high rainfall and Rahad conditions, others (Wad Baku, Farhoda, Gesheish and Wad Fahal) were quite stable under unfavorable conditions, and could be grown under low rainfall conditions of North Gedarif.

CONCLUSION

Based on the results of this study, it could be concluded that the genotypes Mugod,Tabat, Wad-Ahmed, Gadambalia bloom, Safra and Tetron were high yielding and stable under favorable environment, and could be grown under high rainfall and Rahad irrigation conditions. Genotypes Wad Baku, Farhoda, Gesheish and Wad Fahal were quite stable under unfavorable conditions, and could be grown under low rainfall conditions of North Gedarif. Further testing of the unsuitable genotypes is necessary for further breeding manipulations. Both parametric and non- parametric approaches of stability analysis (Eberhart and Russell as well as AMMI) agreed in identifying stable genotypes over different environments.

ICRISAT (International Crops Research Institute for the Semi – Arid Tropics) and FAO (Food and Agriculture Organization of the United Nations ).1996. Part I, Sorghum, pp. 5-27. In: The World Sorghum and Millet Economics: Facts, Trends and Outlook. Rome.

Eighteen sorghum genotypes were evaluated over three consecutive seasons (2009,2010,and 2011) at three locations ( Rahad Research farm of the Agricultural Research Corporation (ARC), Sudan, Gedarif Research Station farm North Gedarif and South Gedarif region). Both experiments conducted in North and South Gedarif were rainfed, while that conducted at Rahad station was irrigated. A randomized complete block design with four replicates was used .The objective was to estimate the general performance, genetic variability and interrelationships of grain yield and its components. Data were collected on days to 50% flowering, plant height, number of heads/m², head length (cm), head width (cm),1000 seed weight (g) and grain yield (kg/ha). The study found that there were highly significant differences among genotypes in all the characters studied except head width in season 2011.The early maturing genotypes were Milo (59-64 days), Gesheish (60-67 days) , AG-8 (59-65 days) and Butana (62-68 days), an indication that these genotypes would fit quite well in short rainy seasons of North Gedarif environment. The late maturing genotypes were Tabat (68-83days),Wad Ahmed (69-83 days), Faki Mustahi (68-88 days) and Tetron (73-88 days) which were suitable to be grown under Rahad and South Gedarif environments. The highest grain yields (kg/ha) were exhibited by the genotypes Butana (735 kg/ha), Wad Ahmed (2572 kg/ha), and Mugod (2545 kg/ha). Grain yield was positively and highly significantly correlated with head width (0.65**) and number of heads/m² (0.46**) .Accordingly, the simple linear correlation and path coefficient analysis indicated that head width and number of heads/m² could be used as potential selection criteria in breeding programs for developing high yielding sorghum genotypes.

INTRODUCTION

Sorghum (Sorghum bicolor (L.) Moench) is one of the major cereal crops of the semi-arid tropics. It is the fifth most important cereal crop of the world. Major producers of sorghum in the world are USA, India, Nigeria, China, Mexico, Sudan and Argentina. Twenty one percent of the world sorghum area is in India. In the Sudan, sorghum is the most widely produced and consumed cereal crop. It is utilized in various forms as stable food for humans, feed for animals and contributes about 70% of total grain produced in the country. It ranks first in total area cultivated as well as total tonnage produced. However, the areas as well as production vary year after year due to many biotic, abiotic, technical and policy factors. The area is reported to be 4-8 million hectares with an average of 5.5 million/ha, about 90% of it is under rain; while total grain production varies between 3-4.5 million tonnes with an average of 0.6 tonnes/ha. Of the abiotic factors limiting sorghum productivity, rainfall stands out as the most important factor. The climatic change seriously affected the traditional sorghum growing areas of northern Gedarif, Gezira, Sennar, White Nile States as well as northern parts of Kordofan and Darfur States. This area is estimated to be > 50% of the total sorghum production area. (Elzein et al, 2008).

In these dry areas (250 mm– 400 mm), farmers used to grow their own local sorghums, which are low yielders and suffer drought stress at almost all stages of crop growth. The outcome is either low yield or straw and chaff. In fact, sorghum is loosing ground in these important areas. The improved, medium maturing, high yielding varieties and hybrids such as Feterita , Wad Ahmed , Hageen Dura-1 and Tabat require 550 mm– 650 mm which is not available and accordingly these varieties/hybrids, were not recommended for these low rainfall regions. (Osman and Mahmoud, 1992). The short maturing varieties released earlier by ARC, such as Umbenien 7, 11, 22, feterita Maatuog, etc, were out of cultivation due to their poor grain quality, small seed, pigmented seed coat and hence low market value.

Progress in plant breeding depends on the extent of genetic variability present in a population that permits effective selection procedures, based on locally adapted land races (Swarp and Chaugale,1962). Therefore, the first step in any plant breeding program is the study of genetic variability, which cannot easily be measured. The ultimate objective of most sorghum breeding programs is to improve yield which is genetically a complex character, that requires a reasonable level of genetic diversity (Sprague, 1966).

Correlation studies are important in breeding programs, as they give information on direction and magnitude of association between different traits. This could be utilized to select for one character indirectly by selection for another one (Sharaan and Ghallab,1997). One of the objectives of the sorghum breeding program in Agricultural Research Corporation (ARC) of the Sudan is to increase productivity and sustainability of sorghum production in irrigated and low rainfall regions of the country and thereby making better use of natural resources.

The present study consists of eighteen sorghum genotypes to be evaluated under different environments (rainfed and irrigation conditions) to contribute to sorghum improvement in the Sudan. The objectives of this study were to evaluate the performance for yield potential, the extent of genetic variability, interrelationships in nine different growing environments of sorghum in the Sudan.

MATERIALS AND METHODS

Location

The experiments were conducted over three consecutive seasons (2009, 2010, and 2011) at three locations,viz. Rahad Research Farm, North and South Gedarif regions of the Gedarif Research Station farm.The three locations lied within the central clay plain of the Sudan characterized by heavy, alkaline clay soil, with a pH of around 8.5 and poor in nitrogen and organic matter.

Plant material

Eighteen accessions of sorghum collected from Gedarif and from the gene bank (Wad Medani) were used in this study. Five of these accessions (Wad-Ahmed, Tabat, Butana, Bashayer and Arffagadamak-8). were released varieties and 13 local land races preferred by farmers Korakollo, Mugod, Saffra, Wad-Bako, Tetron, Faki-Mustahi, Farhoda, Gadambalia bloom, Ajeb-seido, Arafah, Gesheish,Wad-fahal and Milo.

Cultural practices

The standard cultural practices adopted for sorghum production at ARC were followed. Land was prepared by disc ploughing, disc- harrowing, leveling and ridging in irrigated site and by wide level disc in rain-fed sites. Treatments were laid out in a randomized complete block design with four replicates in the different locations and seasons.

Sowing was done in the first week of July under irrigation and the first to the third week of July under rainfed conditions depending on the onset of rainfall. Under irrigation, the entries were sown in five rows, 5 m long on ridges; 0.8 m apart at 0.3 m intra - row spacing and thinned to two seedlings per hill. Under rain fed conditions, they were also sown in five rows 5 m long, on flat; 0.8 m apart at 0.2 m intra row spacing and thinned to two seedlings per hill. Urea at the rates of 80 kg and 40 kg /fed was applied under irrigation and rain-fed sites, respectively, as recommended by the ARC. The crop was irrigated every two weeks or whenever necessary and irrigation was withheld three weeks before harvest.

In irrigated and rainfed experiments, assessments were made in the central three rows of the plot discarding one row or more on each side. The data were collected on days to 50% flowering, plant height, number of heads/m², head length (cm),head width (cm),1000 seed weight (g) and grain yield (kg/ha).

Statistical analysis

Analysis of variance was performed for each season; location and combined to test for significant differences among genotypes. Means for seasons were used to compute simple linear correlation coefficients between all possible combinations. The path coefficients procedure was used in order to partition correlation coefficients between grain yield and its components which is divided into direct and indirect effects.

RESULTS AND DISCUSSION

Mean performance

Days to 50%flowering

This trait is used as an earliness index. Across locations, it showed significant differences among genotypes under the three locations (Table 1).The highest general mean was observed at season 2010 while the lowest general mean was obtained in season 2011. The range for days to 50% flowering was 59 (Milo) to 83 (Wad Ahmed) in 2009, from 60 (Gesheish) to 88 days (Faki Mustahi and Tetron) in 2010, and from 59 (AG-8) to 74 days (Tetron) in 2011.

Identifying early and medium maturing genotypes is important for choosing genotypes to suit the different growing environments (irrigation and rainfed). Hence, from these findings, the early maturing genotypes were Milo, Gesheish , AG-8 and Butana, an indication that these lines would fit quite well in short rainy seasons, i.e suitable for North Gedarif environment, while the late maturing ones were Tabat, Wad Ahmed, Faki Mustahi and Tetron which were suitable to grow under Rahad and South Gedarif environments (Table 1). These findings were in agreement with those of Abdalla et al. (2009), who reported that lines AG-8 and AG-15 were 18 days and 14 days earlier than Wad-Ahmed. Elzein et al. (2008) found a wide range of variability in days to 50% flowering

Plant height (cm)

Development of short, medium genotypes is important for any plant breeding program, because these genotypes will be suitable for mechanical harvesting and for resistance to lodging. Across locations, plant height showed significant differences among genotypes under the three locations (Table 1). The highest general means were observed in season 2010, while the lowest general means were observed in season 2011. The range for plant height was 112 cm (Tabat) to 189 cm (Wad Baku) in 2009, from 137cm (Butana) to 216 cm (Tetron) in 2010 and from 97 cm (Bashaiyer and AG-8) to 139 cm (Tetron) in 2011.Thus, in this study, the short genotypes were Tabat, Butana , Bashaiyer and AG-8, while the tall genotypes were Wad Baku and Tetron (Table 1). From these results, tall genotypes such as Wad Baku and Tetron are not appropriate for drought areas such as North Gedarif , because they were susceptible to drought, while Butana and Bashayier were suitable to grow under North Gedarif conditions. These findings were in agreement with Elasha et al (2011) , who found significant

differences (P< 0.01) between the entries in their plant height. Also Bushara (1999) , recorded highly significant differences in plant height among

This character is an indicator for high grain yield. Good crop establishment will result in increasing number of heads/m² which lead to an increase in grain yield. Across locations, this trait showed highly significant differences among the genotypes in the three locations (Table 2). The highest general mean was observed in season 2010, while the lowest

general mean was observed in season 2011. The range for this trait was from 6 (Wad Fahal, Mugod, Tabat) to 11 (Wad Ahmed) in season 2009, from 8 (Wad Fahal) to 13 (Wad Baku) in season 2010, from 5 (Arafa and Faki Mustahi) to 9 (Korakollo) in season 2011 (Table 2 ). In this study, the highest numbers of heads/m² were obtained by the genotypes Wad Ahmed, Wad Baku, and Korakollo, while the lowest number of heads/m² were obtained by the genotypes Wad Fahal, Tabat and Arafa. This is because Wad Fahal and Arafa are late in maturity and some plants failed to produce heads due to moisture stress.

Head length (cm)

Highly significant differences among genotypes were observed for this trait under the three locations (Table 2). Across locations, the highest general mean(19 cm) was observed in season 2010, while the lowest general mean (14 cm) was observed in season 2011. The range for this trait was from 13 cm (Mugod, Farhoda and Wad Baku) to 24 cm (Faki Mustahi) in season 2009, from 12 cm (Mugod) to 27 cm (Tetron) in season 2010, from 12 cm (Safra and Farhoda) to 18 cm (Tabat) in season 2011. In this study, the longest heads were obtained by the genotypes Faki Mustahi ,Tetron, and Tabat ,while the shortest heads were obtained by the genotypes Mugod and Safra (Table 2 ).This result agreed with that reported by Elasha et al (2011),who found that during both seasons at the irrigated and the rainfed sites, there were significant differences (P< 0.01) between the entries in their panicle length.

Table 2. Means of number of heads/m² and head length (cm) for 18 Sorghum genotypes grown at North Gedarif (NG),South Gedarif (SG),and

Rahad (RH), season 2009, 2010, 2011.

No. of heads/m²

Head length (cm)

Genotypes

2009

2010

2011

Mean

2009

2010

2011

Mean

Korakollo

8

11

9

9

17

18

15

16

Mugod

6

11

7

8

13

12

14

13

Safra

8

11

7

8

17

14

12

14

Wad Baku

8

13

8

9

13

14

14

13

Tetron

7

10

6

8

17

27

17

20

Faki Mustahi

10

9

5

8

24

26

17

22

Farhoda

7

10

6

8

13

13

12

12

Gadambaliabloom

9

11

6

8

16

17

13

15

Ajeb seido

9

11

8

9

15

19

13

15

Arafa

8

9

5

7

20

19

15

18

AG-8

10

10

7

9

15

18

14

15

Butana

9

10

7

9

23

24

15

20

Bashayier

7

11

7

8

22

20

14

18

Tabat

6

9

7

7

20

22

18

20

Wad ahmed

11

11

8

10

17

18

15

16

Gesheish

7

11

8

8

19

17

14

16

Wad Fahal

6

8

7

7

21

20

14

18

Milo

9

11

7

9

16

17

13

15

Mean

8

10

7

8

18

19

14

17

CV%

29.6

23.9

8.5

20.6

12.4

22.0

14.6

16.6

SE±

0.67

0.72

0.71

6.7

0.62

1.18

0.80

0.86

**

**

**

**

**

**

** Significant at 0.01 of probability level.

Head width (cm)

Across locations, this trait showed highly significant differences among genotypes except for season 2011. The highest general mean (5cm) was observed in season 2010, while the lowest general mean (3cm) was observed in season 2011(Table 3). The range for head width varied from 3 cm (Faki Mustahi) to 4 cm (Wad Fahal), from 4 cm (Faki Mustahi) to 6cm (Butana and Gesheish), from 3cm (Bashaiyer) to 4cm (Mugod) in seasons 2009, 2010 , and 2011, respectively (Table 3). From this study, genotype Mugod had the largest head width coupled with the longest head length. This means that tall, late maturing genotypes are not suitable for drought areas such as North Gedarif but suitable for South Gedarif and Rahad environments.

1000 seed weight (g)

Across locations, highly significant differences among genotypes were observed for this trait.The highest general mean (31g) was observed in season 2010,while the lowest general mean (25 g) was obtained in seasons 2009 and 2011(Table 3). The range for 1000 -seed weight (g) varied from 18 g (Butana) to 36 g (Mugod), from 25g (Tetron and Wad Ahmed) to 42g (Wad Fahal) , from 21g (Wad Ahmed) to 31g (Wad Fahal) in season 2009, 2010, and 2011, respectively. Hence, in this study, the highest 1000 seed weight was obtained by the genotypes Mugod and Wad-fahal,while the lowest 1000 seed weight (g) was exhibited by the genotypes Butana , Tetron and Wad Ahmed (Table 3). These findings agreed with those reported by Geremew (1993), who recorded a wide range of variability in 1000 seed weight. From these findings genotypes Mugod and Wad Fahal had large seed size compared to the genotypes Butana,Wad Ahmed and Tetron with medium seed size. Tall, late maturing genotypes with large to medium seed size such as Tetron are not suitable to grow under North Gedarif conditions because they need higher rainfall.

Across locations, this trait showed highly significant differences among the genotypes. The highest general mean (1973 kg/ha) was observed in season 2010, while the lowest general mean (450 kg/ha) was obtained in season 2009 (Table 4).The range for this trait was from 225 kg (Farhoda) to 735 kg (Butana) in season 2009, from 1408 kg (Faki Mustahi) to 2572 kg (Wad Ahmed) in season 2010, from 862 kg (Faki Mustahi) to 2545 kg (Mugod) in season 2011, respectively. Hence, in this study the highest grain

yields (kg/ha) were exhibited by the genotypes Butana, Wad Ahmed, and Mugod. Similar results were reported by Elasha et al .(2011);Elzein et al, 2008, and Abdalla et al (2009). While the lowest grain yields were obtained by the genotypes Farhoda and Faki Mustahi (Table 4).This study indicated that Butana was an early maturing and short genotype which is suitable for North Gedarif conditions ,while medium or tall genotypes that are late maturing, coupled with high grain yield such as Mugod and Wad Ahmed were suitable for growing under irrigation and high rainfall.

Across seasons, grain yield showed highly significant differences (Table 4). The highest general mean (1563 kg/ha) for this trait was obtained at Rahad location, while the lowest general mean (803 kg/ha ) was observed at North Gedarif. The range for this trait was from 409 kg (Wad Fahal) to 1129 kg (Arafa), from 916 kg (Faki mustahi) to 2572 kg (Mugod) , and from 1060 kg (Wad Baku) to 2120 kg (Wad Ahmed) for North Gedarif , South Gedarif and Rahad , respectively. Hence, in this study the highest grain yields were exhibited by the genotypes Mugod at South Gedarif and Wad Ahmed in Rahad location ,while the lowest grain yield were obtained by the genotypes Wad Fahal , Faki Mustahi and Wad Baku at North Gedarif, South Gedarif and Rahad, respectively. This is because all of them are tall and late maturing genotypes and they produce small seeds under drought spell conditions. From this finding, genotype Tetron was late maturing, tall ,with medium seed size and high grain yield, genotype Gesheish was early maturing and has low yield, while genotype Mugod is medium maturing, having medium height with big seed size and high yielding , while Faki Mustahi was a late maturing genotype with a small seed size and has low yield.

Grain yield was positively , and highly significantly correlated with head width (0.65**), number of heads/m² (0.46**) , and 1000-seed weight (0.32*). It was positively and non-significantly correlated with days to 50% flowering (Table 5). Similar results were reported by Bittinger et al. (1981) and Elagib (1999) who found that grain yield was positively correlated with days to 50% flowering and 1000-grain weight. Also, positive association between grain yield and days to 50% flowering was reported by many authors; Liang et al. (1969) in sorghum and Umakanth et al. (2001) found

that correlation coefficients were moderate to high for days to anthesis, plant height,1000-seed weight, number of plant/m² and number of heads/m². Liang et al. (1969) found that 1000-grain weight was significantly correlated with grain yield. Shukla (1966) Chigwe (1984) found that 1000-seed weight was significantly correlated with grain yield under dry conditions in all maturity groups. Hadjichrislodoulu (1990) and Krishnasamy (1986) found that days to 50%

flowering showed a significant positive correlation with plant height in some hybrids. Also, a positive correlation was reported by Rana et al. (1984) who found an association between fodder yield and days to 50% flowering.Grain yield was significantly and negatively correlated with number of plants at establishment/m² but negatively and non-significantly correlated with head length and plant height. Plant height was positively and highly significantly correlated with one thousand seed weight (0.48**), head width (43**), number of heads /m² (0.40**), and number of plants/m² (0.35**).These results indicated that selection for these traits may be effective in improvement of grain yield, in addition, these findings indicated that tall plants possess heavier heads than short ones.

One thousand seed weight was positively and highly significantly correlated with head width (0.59**) and plant height (0.48**) (Table 5), but positively and not significantly correlated with number of heads/m² and days to 50% flowering.1000-seed weight was negatively correlated with number of plants/m² and head length, Significant and positive correlations of 1000-seed weight with plant height were reported by Ezeaku and Mohamed (2006). Hence, in this study, head width, number of heads/m² and 1000-grain weight had strong correlation with grain yield (0.65**), (0.46**), and (0.32*), respectively, while it was negatively correlated with plant height, number of plants/m², and head length. The positive and significant association of grain yield with head width and number of heads/m² was mainly due to their positive direct effect with negligible indirect effects through other characters. This suggested the direct use of these two characters as selection criteria.

Table 5. Simple linear correlation coefficients among various pairs of 8 characters of sorghum genotypes combined over three seasons (2009,2010,2011) and three locations (North Gedarif, South Gedarif and Rahad).

The relatively large, positive and significant simple linear correlation coefficient between grain yield and number of heads/m² was (0.46**) .The positive direct effect of number of heads/m² on grain yield was the highest (0.47) (Table 6). The highest positive direct effect on grain yield was exhibited by head width (0.33). Its indirect effect through number of heads/m² is large while too small through the other characteristics. This suggested the use of this character as a selection criterion for the improvement of grain yield. Its indirect effects on grain yield were negligible through the other traits.

The relatively small, negative simple linear correlation between grain yield and head length (-0.02) is explained via the negative direct effect of head length on grain yield (-0.12) ,so it is difficult to recommend this character as a selection criterion for yield (Table 6).

Head width was highly significantly and positively correlated with grain yield (o.65**) (Table 6),such strong association is explained via the high positively direct effect of head width on grain yield(0.33). Low negative indirect effect were observed via plant height, head length, and 1000-seed weight. Also, it had low positive indirect effect on grain yield via number of plants/m², number of heads/m² and days to 50% flowering (Table 6).

In this study, correlation and path analysis may measure two different aspects. Hence, the study of correlation alone does not give accurate indications of yield association. For example, in this study correlation between days to 50% flowering and grain yield was very small (0.13). This means that this character had no in influence on grain yield, but the path analysis expressed days to 50% flowering as an important trait influencing yield.

From the present study, the direct effect of the tested traits on grain yield indicated that among yield components head width and number of heads/m² had the highest correlation coefficient with grain yield. These traits also showed a positive direct effect on grain yield and therefore, these characters may be considered as selection criteria for developing high yielding sorghum genotypes.

X1: Days to 50% flowering,X2:Plant height,X3:number of plants at establishment/m2,X4:number of heads/m2,X5: Head length,X6: Head width,X7:1000 SW(g).

CONCLUSION

Based on the results of this study, it could be concluded that a wide range of genetic variability was observed among sorghum genotypes for most of the characters studied. The early maturing genotypes were Milo, Gesheish, AG-8 and Butana, an indication that these genotypes would fit quite well in short rainy seasons, which were suitable for North Gedarif environment, while the late maturing ones were Tabat,Wad Ahmed, Faki Mustahi and Tetron which were suitable to grow under Rahad and South Gedarif environments. The highest grain yields ( kg/ha) were exhibited by the genotypes Butana (735 kg/ha),Wad Ahmed (2572 kg/ha),and Mugod (2545kg/ha). Simple linear correlation and path coefficient analysis indicated that head width and numbers of heads/m² could be used as potential selection criteria in breeding programs for developing high yielding sorghum genotypes.

Combining ability for grain yield and yield components in local inbred lines and introduced open pollinated varieties of maize (Zea mays L).

ABSTRACT

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.

Tallness is not a good character in grain maize production, since tall maize plants tend to be susceptible to stem and root lodging. Highly significant differences for tallness were detected among the studied parents with the general mean being of 144.4 cm. The trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. In the studied parents mean plant height ranged between 131.1 cm for L7 to 158 cm for T3 which was the tallest and latest parent across locations (Table 2). The crosses mean varied from 135.1 cm for (T3 x L7) to 155.9 cm for (T2 x L1).The tallest hybrids across locations were T4 x L6 and T4 x L3 (154 cm) (Table 3).

Table 3. Performance of 28 crosses for the measured traits in maize at the two locations combined, season 2008.

The results indicate that crosses were later than their parents. Also, the taller crosses were late maturing than short ones. Generally, the crosses were taller than their parents which suggested some degree of hybrid vigor.

Ear length trait is an important selection index for grain yield in maize. The ear length means of parents, as expected, were found to be shorter than those of the crosses at the two sites, with the general mean of 13.5 cm. The parents mean ranged between 12.7 cm for L7 to 15 cm for L2 (Table 2). The crosses mean varied from 11.9 cm for (T3 x L4) to 16.1 cm for (T3 x L5). However, long ear length were recorded for crosses T1 x L7 (15.2 cm), and T2 x L6 (14.6cm) (Table 3).Vedia and Claure (1995) found that ear length was the most important yield component and when used as a selection index genetic gain in recurrent selection reached 9.94% for yield and 5.75% for the ear traits. Therefore, any increase in ear length would be expected to increase number of kernels/row and hence increase grain yield.

Ear diameter is a good indicator of the number of kernel rows/ear. The mean of ear diameter across sites for parents ranged between 3.4 cm for L6 and L7 to 4.1 cm for L4 (Table 2). Among the crosses, the large ear diameter ranged from 3.3 cm for T3 x L6 to 4.0 cm for T2 x L1. The crosses which had a big ear diameter were T3 x L3 and T4 x L6 (3.8cm) (Table 3). This result was in agreement with the findings of Tracy (1990) who found that the maize hybrids with high yield had more ears/plant, longer ears and a better ear shape and row configuration.

The mean of one hundred kernels weight for parents was 21.4 g, and it ranged between 19.9 g for L2 to 22.8 g for T3 (Table 2). Among the crosses, the mean was 21.3 g. The best crosses which obtained the highest kernel weight were T1 x L2 and T4 x L3 (23.2) followed by T2 x L3 (22.8 g) (Table 3).

Yield is a polygenic character is influenced by the fluctuating enviro-nment. Moreover, it is a complex trait depending on many components (Sharaan and Ghallab, 1997). In this study, there was a considerable amount of variability among the genotypes for this trait. The studied parents in the two locations showed a general mean of 2.4 t/ha. The parents means ranged between 2.12 t/ha for L4 to 2.93 t/ha for T4 (Table 2), while, the crosses means ranged between 2.0 t/ha for (T2 x L5) to 3.55 t/ha for (T2 x L7) (Table 3). Most of the crosses (19 hybrids) had significantly higher mean grain yield than the overall mean. It is of interest to mention that the top ranking and the best yielder hybrids were T1 x L2 (3.4 t/ha), T2 x L1 (3.3 t/ha), T4 x L4 (3.3 t/ha), T1 x L1 (3.30 t/ha) and T4 x L3 (3.1 t/ha). These results agreed with those of Khalafalla and Abdalla (1997), who pointed to the fact that hybrids (crosses) produce higher grain yields than the open pollinated varieties due to the good performance of hybrids under Sudan conditions.

Combining ability

The breeding method to be adopted for improvement of a crop depends primarily on the nature of gene action involved in the expression of quantitative traits of economic importance. Combining ability leads to identification of parents with general combining ability effects and in locating cross combining showing high specific combining ability effects. In this study the ratio of GCA to SCA mean variance for most traits was less than one, suggesting that the inheritance of these traits was due to non additive gene action, with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and largely

This result indicates that dominance and epistatic interaction effects seemed to be predomint for this trait and therefore heterosis breeding may be gratifying. The good combiner parents, those having negative GCA effects in Medani, for 50% days to tasseling were L5 followed by T4, T1 and L7, indicating earliness for flowering time, while, the latest, having positive GCA effect was T3 (Table 5).The earliest crosses having negative SCA effects were T3 x L6, T2 x L7 and T2 x L4, while, the latest crosses were T2 x L5, T4 x L5 and T4 x L4 (Table.6).

The earliest parent in Mutaq was L7 (Table 5) and the earliest crosses were T2 x L4, T4 x L4 and T3 x L4 (Table 6). Common parents across locations that contributed to earliness were T4 and L5. The latest were L6 followed by T3 and T2 (Table 5). Parent L4 had good contribution for earliness to their hybrids progeny across locations.

Thus, the inbred lines which exhibited good general combining ability for at least one character can be used for development of early maturity and high grain yield. The contribution of the total variance for general and specific combining ability for this trait differs from location to another, but SCA was high in both locations (50.4% and 71.7%) compared with GCA which indicates that this trait is controlled by additive gene action (Figs 1 and 2).

Trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. Only three top cross hybrid parents in Medani have negative GCA effects for plant height, i.e., L7, L3 and T3; they were best combiners for short plant type. Tallness which is an undesirable trait is shown by parents L1, L2 and T1 (Table 5). Crosses having negative SCA effects and consequently short plant type were T4 x L2, T1 x L4 and T2 x L4, while, tall hybrids with positive SCA effects were T3 x L1, T2 x L5 and T4 x L5 (Table 6).

The best combiners for the short plant type with negative GCA in Mutaq were L7, L6 and L2 while, the taller parents with high positive GCA effects were L5 and L1 (Table 5). Among the crosses the shortest hybrids were T2 x L4, followed by T3 x L5 and the tallest hybrids were T2 x L5 and T3 x L4 (Table 6). This showed that, there is a relationship between late flowering and tall plant type. This is quite obvious among the hybrids such as T3 x L1 and T3 x L4. Contribution for this trait is higher in crosses (80% and 53%) compared to parents (20% and 40%) at the two locations (Figs 1 and 2). The earliness and shortness are desirable traits especially under rainfed conditions for better water use efficiency and the escape of drought and avoidance of late season infestation with stem borer.

Ear length is a good index for higher grain yield, therefore any increase in ear length would be expected to increase number of kernels/row and hence directly improve grain yield. In Medani site, the long ear length parents having a positively significant GCA effects such as L5, L7 and T1, while parents showing the short ear length were L4 and L2 (Table 5). The best crosses for this trait having a positive SCA effects and hence the longest ear length were T2 x L5 and T4 x L7. On the other hand the best combiners in Mutaq were L7 and T3 (Table 5), while the best crosses were T1 x L1 and T4 x L4 (Table 6). In the two locations, the best contribution was (73% and 65.9%) obtained by SCA compared with (27% and44.1) for GCA (Figs1 and 2). These results emphasized that ear length has a direct effect for improving grain yield. This is in agreement with the finding of Vedia and Claure (1995) who found that ear aspect was the most important yield component.

Based on GCA estimates, the best combiners for ear diameter and length in Medani are L1 and L5, while best crosses were T1 x L2, T3 x L5 and T3 x L7. The good combiners in Mutaq site are L2, L3 and L4, while the best crosses are T3 x L4, T4 x L1 and T1 x L5 (Tables 5 and 6). A higher contribution among this trait is obtained by SCA (55.9% and 65.9%) in both locations compared with GCA (Figs 1and 2).

Favorable GCA values were given by T1 and L3 as the good combiners for kernel weight in Medani and the best crosses were T4 x L7 and T2 x L7. Among the studied parent material in Mutaq, only three parents have positive GCA effects (L3, L4 and T3).

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 1. Parent contribution of the maize GCA and SCA to the total variance

of yield and its componets at Medani, season 2008.

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 2. Parent contribution of the maize GCA and SCA to the total variance of yield and its componets at Mutaq, season 2008.

The best crosses were shown by T1 x L4 and T3 x L4 (Tables 5 and 6). The higher average contribution was given by the SCA (50.8% and 61) compared with the GCA at two locations (Figs 1and 2). This indicted that the inheritance of this trait was controlled by non additive gene effects.

At Medani site, all the results depicted in Table 5 showed that the parents differ considerably with respect to estimate of GCA effects for grain yield. The parents having positive GCA effects were T1 followed by L4 and L6. Parents having negative GCA effects were L2 and L6. The best crosses having positive SCA effects were T3 x L3 followed by T4 x L5 and T1 x L2. Negative SCA effects were shown by T3 x L4, T2 x L2 and T1 x L4 (Table 6). The higher combiner in Mutaq, were L2, L1 and L4. The best crosses were T3 x L5, T1 x L5 and T4 x L5, while negative SCA effects were shown by T1 x L3, T2 x L5 and T4 x L3 (Tables 5 and6). The great contribution was given by SCA (62.4% and 62%) compared with GCA at the two locations (Figs1 and 2).

General combing ability variance for grain yield is greater than the mean square for specific combining ability indicating the importance of additive gene action in controlling grain yield. This finding is in agreement with that of Barakat and Osman (2008) who found GCA effects are larger than SCA effects for grain yield indicating that the additive genetic variance is a major source of variations responsible for inheritance of grain yield.

CONCLUSION

The ratio of general combining ability variance for grain yield was greater than specific combining ability indicating the importance of additive gene action in controlling this trait hence the good combiner parent for grain yield across locations was L4 so it could be used in recurrent selection. Also enormous variability was detected in the studied population which makes cyclic selection more effective. The best cross was T4 x L5 indicating that dominance and epesitic interaction seemed to be predomint, hence, higher heterosis gratified and recommended cross T4 x L5 for future testing in multi-locations trials for commercial utilization in order to be released as a hybrid.

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.

Tallness is not a good character in grain maize production, since tall maize plants tend to be susceptible to stem and root lodging. Highly significant differences for tallness were detected among the studied parents with the general mean being of 144.4 cm. The trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. In the studied parents mean plant height ranged between 131.1 cm for L7 to 158 cm for T3 which was the tallest and latest parent across locations (Table 2). The crosses mean varied from 135.1 cm for (T3 x L7) to 155.9 cm for (T2 x L1).The tallest hybrids across locations were T4 x L6 and T4 x L3 (154 cm) (Table 3).

Table 3. Performance of 28 crosses for the measured traits in maize at the two locations combined, season 2008.

The results indicate that crosses were later than their parents. Also, the taller crosses were late maturing than short ones. Generally, the crosses were taller than their parents which suggested some degree of hybrid vigor.

Ear length trait is an important selection index for grain yield in maize. The ear length means of parents, as expected, were found to be shorter than those of the crosses at the two sites, with the general mean of 13.5 cm. The parents mean ranged between 12.7 cm for L7 to 15 cm for L2 (Table 2). The crosses mean varied from 11.9 cm for (T3 x L4) to 16.1 cm for (T3 x L5). However, long ear length were recorded for crosses T1 x L7 (15.2 cm), and T2 x L6 (14.6cm) (Table 3).Vedia and Claure (1995) found that ear length was the most important yield component and when used as a selection index genetic gain in recurrent selection reached 9.94% for yield and 5.75% for the ear traits. Therefore, any increase in ear length would be expected to increase number of kernels/row and hence increase grain yield.

Ear diameter is a good indicator of the number of kernel rows/ear. The mean of ear diameter across sites for parents ranged between 3.4 cm for L6 and L7 to 4.1 cm for L4 (Table 2). Among the crosses, the large ear diameter ranged from 3.3 cm for T3 x L6 to 4.0 cm for T2 x L1. The crosses which had a big ear diameter were T3 x L3 and T4 x L6 (3.8cm) (Table 3). This result was in agreement with the findings of Tracy (1990) who found that the maize hybrids with high yield had more ears/plant, longer ears and a better ear shape and row configuration.

The mean of one hundred kernels weight for parents was 21.4 g, and it ranged between 19.9 g for L2 to 22.8 g for T3 (Table 2). Among the crosses, the mean was 21.3 g. The best crosses which obtained the highest kernel weight were T1 x L2 and T4 x L3 (23.2) followed by T2 x L3 (22.8 g) (Table 3).

Yield is a polygenic character is influenced by the fluctuating enviro-nment. Moreover, it is a complex trait depending on many components (Sharaan and Ghallab, 1997). In this study, there was a considerable amount of variability among the genotypes for this trait. The studied parents in the two locations showed a general mean of 2.4 t/ha. The parents means ranged between 2.12 t/ha for L4 to 2.93 t/ha for T4 (Table 2), while, the crosses means ranged between 2.0 t/ha for (T2 x L5) to 3.55 t/ha for (T2 x L7) (Table 3). Most of the crosses (19 hybrids) had significantly higher mean grain yield than the overall mean. It is of interest to mention that the top ranking and the best yielder hybrids were T1 x L2 (3.4 t/ha), T2 x L1 (3.3 t/ha), T4 x L4 (3.3 t/ha), T1 x L1 (3.30 t/ha) and T4 x L3 (3.1 t/ha). These results agreed with those of Khalafalla and Abdalla (1997), who pointed to the fact that hybrids (crosses) produce higher grain yields than the open pollinated varieties due to the good performance of hybrids under Sudan conditions.

Combining ability

The breeding method to be adopted for improvement of a crop depends primarily on the nature of gene action involved in the expression of quantitative traits of economic importance. Combining ability leads to identification of parents with general combining ability effects and in locating cross combining showing high specific combining ability effects. In this study the ratio of GCA to SCA mean variance for most traits was less than one, suggesting that the inheritance of these traits was due to non additive gene action, with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and largely

This result indicates that dominance and epistatic interaction effects seemed to be predomint for this trait and therefore heterosis breeding may be gratifying. The good combiner parents, those having negative GCA effects in Medani, for 50% days to tasseling were L5 followed by T4, T1 and L7, indicating earliness for flowering time, while, the latest, having positive GCA effect was T3 (Table 5).The earliest crosses having negative SCA effects were T3 x L6, T2 x L7 and T2 x L4, while, the latest crosses were T2 x L5, T4 x L5 and T4 x L4 (Table.6).

The earliest parent in Mutaq was L7 (Table 5) and the earliest crosses were T2 x L4, T4 x L4 and T3 x L4 (Table 6). Common parents across locations that contributed to earliness were T4 and L5. The latest were L6 followed by T3 and T2 (Table 5). Parent L4 had good contribution for earliness to their hybrids progeny across locations.

Thus, the inbred lines which exhibited good general combining ability for at least one character can be used for development of early maturity and high grain yield. The contribution of the total variance for general and specific combining ability for this trait differs from location to another, but SCA was high in both locations (50.4% and 71.7%) compared with GCA which indicates that this trait is controlled by additive gene action (Figs 1 and 2).

Trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. Only three top cross hybrid parents in Medani have negative GCA effects for plant height, i.e., L7, L3 and T3; they were best combiners for short plant type. Tallness which is an undesirable trait is shown by parents L1, L2 and T1 (Table 5). Crosses having negative SCA effects and consequently short plant type were T4 x L2, T1 x L4 and T2 x L4, while, tall hybrids with positive SCA effects were T3 x L1, T2 x L5 and T4 x L5 (Table 6).

The best combiners for the short plant type with negative GCA in Mutaq were L7, L6 and L2 while, the taller parents with high positive GCA effects were L5 and L1 (Table 5). Among the crosses the shortest hybrids were T2 x L4, followed by T3 x L5 and the tallest hybrids were T2 x L5 and T3 x L4 (Table 6). This showed that, there is a relationship between late flowering and tall plant type. This is quite obvious among the hybrids such as T3 x L1 and T3 x L4. Contribution for this trait is higher in crosses (80% and 53%) compared to parents (20% and 40%) at the two locations (Figs 1 and 2). The earliness and shortness are desirable traits especially under rainfed conditions for better water use efficiency and the escape of drought and avoidance of late season infestation with stem borer.

Ear length is a good index for higher grain yield, therefore any increase in ear length would be expected to increase number of kernels/row and hence directly improve grain yield. In Medani site, the long ear length parents having a positively significant GCA effects such as L5, L7 and T1, while parents showing the short ear length were L4 and L2 (Table 5). The best crosses for this trait having a positive SCA effects and hence the longest ear length were T2 x L5 and T4 x L7. On the other hand the best combiners in Mutaq were L7 and T3 (Table 5), while the best crosses were T1 x L1 and T4 x L4 (Table 6). In the two locations, the best contribution was (73% and 65.9%) obtained by SCA compared with (27% and44.1) for GCA (Figs1 and 2). These results emphasized that ear length has a direct effect for improving grain yield. This is in agreement with the finding of Vedia and Claure (1995) who found that ear aspect was the most important yield component.

Based on GCA estimates, the best combiners for ear diameter and length in Medani are L1 and L5, while best crosses were T1 x L2, T3 x L5 and T3 x L7. The good combiners in Mutaq site are L2, L3 and L4, while the best crosses are T3 x L4, T4 x L1 and T1 x L5 (Tables 5 and 6). A higher contribution among this trait is obtained by SCA (55.9% and 65.9%) in both locations compared with GCA (Figs 1and 2).

Favorable GCA values were given by T1 and L3 as the good combiners for kernel weight in Medani and the best crosses were T4 x L7 and T2 x L7. Among the studied parent material in Mutaq, only three parents have positive GCA effects (L3, L4 and T3).

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 1. Parent contribution of the maize GCA and SCA to the total variance

of yield and its componets at Medani, season 2008.

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 2. Parent contribution of the maize GCA and SCA to the total variance of yield and its componets at Mutaq, season 2008.

The best crosses were shown by T1 x L4 and T3 x L4 (Tables 5 and 6). The higher average contribution was given by the SCA (50.8% and 61) compared with the GCA at two locations (Figs 1and 2). This indicted that the inheritance of this trait was controlled by non additive gene effects.

At Medani site, all the results depicted in Table 5 showed that the parents differ considerably with respect to estimate of GCA effects for grain yield. The parents having positive GCA effects were T1 followed by L4 and L6. Parents having negative GCA effects were L2 and L6. The best crosses having positive SCA effects were T3 x L3 followed by T4 x L5 and T1 x L2. Negative SCA effects were shown by T3 x L4, T2 x L2 and T1 x L4 (Table 6). The higher combiner in Mutaq, were L2, L1 and L4. The best crosses were T3 x L5, T1 x L5 and T4 x L5, while negative SCA effects were shown by T1 x L3, T2 x L5 and T4 x L3 (Tables 5 and6). The great contribution was given by SCA (62.4% and 62%) compared with GCA at the two locations (Figs1 and 2).

General combing ability variance for grain yield is greater than the mean square for specific combining ability indicating the importance of additive gene action in controlling grain yield. This finding is in agreement with that of Barakat and Osman (2008) who found GCA effects are larger than SCA effects for grain yield indicating that the additive genetic variance is a major source of variations responsible for inheritance of grain yield.

CONCLUSION

The ratio of general combining ability variance for grain yield was greater than specific combining ability indicating the importance of additive gene action in controlling this trait hence the good combiner parent for grain yield across locations was L4 so it could be used in recurrent selection. Also enormous variability was detected in the studied population which makes cyclic selection more effective. The best cross was T4 x L5 indicating that dominance and epesitic interaction seemed to be predomint, hence, higher heterosis gratified and recommended cross T4 x L5 for future testing in multi-locations trials for commercial utilization in order to be released as a hybrid.

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.

Tallness is not a good character in grain maize production, since tall maize plants tend to be susceptible to stem and root lodging. Highly significant differences for tallness were detected among the studied parents with the general mean being of 144.4 cm. The trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. In the studied parents mean plant height ranged between 131.1 cm for L7 to 158 cm for T3 which was the tallest and latest parent across locations (Table 2). The crosses mean varied from 135.1 cm for (T3 x L7) to 155.9 cm for (T2 x L1).The tallest hybrids across locations were T4 x L6 and T4 x L3 (154 cm) (Table 3).

Table 3. Performance of 28 crosses for the measured traits in maize at the two locations combined, season 2008.

The results indicate that crosses were later than their parents. Also, the taller crosses were late maturing than short ones. Generally, the crosses were taller than their parents which suggested some degree of hybrid vigor.

Ear length trait is an important selection index for grain yield in maize. The ear length means of parents, as expected, were found to be shorter than those of the crosses at the two sites, with the general mean of 13.5 cm. The parents mean ranged between 12.7 cm for L7 to 15 cm for L2 (Table 2). The crosses mean varied from 11.9 cm for (T3 x L4) to 16.1 cm for (T3 x L5). However, long ear length were recorded for crosses T1 x L7 (15.2 cm), and T2 x L6 (14.6cm) (Table 3).Vedia and Claure (1995) found that ear length was the most important yield component and when used as a selection index genetic gain in recurrent selection reached 9.94% for yield and 5.75% for the ear traits. Therefore, any increase in ear length would be expected to increase number of kernels/row and hence increase grain yield.

Ear diameter is a good indicator of the number of kernel rows/ear. The mean of ear diameter across sites for parents ranged between 3.4 cm for L6 and L7 to 4.1 cm for L4 (Table 2). Among the crosses, the large ear diameter ranged from 3.3 cm for T3 x L6 to 4.0 cm for T2 x L1. The crosses which had a big ear diameter were T3 x L3 and T4 x L6 (3.8cm) (Table 3). This result was in agreement with the findings of Tracy (1990) who found that the maize hybrids with high yield had more ears/plant, longer ears and a better ear shape and row configuration.

The mean of one hundred kernels weight for parents was 21.4 g, and it ranged between 19.9 g for L2 to 22.8 g for T3 (Table 2). Among the crosses, the mean was 21.3 g. The best crosses which obtained the highest kernel weight were T1 x L2 and T4 x L3 (23.2) followed by T2 x L3 (22.8 g) (Table 3).

Yield is a polygenic character is influenced by the fluctuating enviro-nment. Moreover, it is a complex trait depending on many components (Sharaan and Ghallab, 1997). In this study, there was a considerable amount of variability among the genotypes for this trait. The studied parents in the two locations showed a general mean of 2.4 t/ha. The parents means ranged between 2.12 t/ha for L4 to 2.93 t/ha for T4 (Table 2), while, the crosses means ranged between 2.0 t/ha for (T2 x L5) to 3.55 t/ha for (T2 x L7) (Table 3). Most of the crosses (19 hybrids) had significantly higher mean grain yield than the overall mean. It is of interest to mention that the top ranking and the best yielder hybrids were T1 x L2 (3.4 t/ha), T2 x L1 (3.3 t/ha), T4 x L4 (3.3 t/ha), T1 x L1 (3.30 t/ha) and T4 x L3 (3.1 t/ha). These results agreed with those of Khalafalla and Abdalla (1997), who pointed to the fact that hybrids (crosses) produce higher grain yields than the open pollinated varieties due to the good performance of hybrids under Sudan conditions.

Combining ability

The breeding method to be adopted for improvement of a crop depends primarily on the nature of gene action involved in the expression of quantitative traits of economic importance. Combining ability leads to identification of parents with general combining ability effects and in locating cross combining showing high specific combining ability effects. In this study the ratio of GCA to SCA mean variance for most traits was less than one, suggesting that the inheritance of these traits was due to non additive gene action, with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and largely

This result indicates that dominance and epistatic interaction effects seemed to be predomint for this trait and therefore heterosis breeding may be gratifying. The good combiner parents, those having negative GCA effects in Medani, for 50% days to tasseling were L5 followed by T4, T1 and L7, indicating earliness for flowering time, while, the latest, having positive GCA effect was T3 (Table 5).The earliest crosses having negative SCA effects were T3 x L6, T2 x L7 and T2 x L4, while, the latest crosses were T2 x L5, T4 x L5 and T4 x L4 (Table.6).

The earliest parent in Mutaq was L7 (Table 5) and the earliest crosses were T2 x L4, T4 x L4 and T3 x L4 (Table 6). Common parents across locations that contributed to earliness were T4 and L5. The latest were L6 followed by T3 and T2 (Table 5). Parent L4 had good contribution for earliness to their hybrids progeny across locations.

Thus, the inbred lines which exhibited good general combining ability for at least one character can be used for development of early maturity and high grain yield. The contribution of the total variance for general and specific combining ability for this trait differs from location to another, but SCA was high in both locations (50.4% and 71.7%) compared with GCA which indicates that this trait is controlled by additive gene action (Figs 1 and 2).

Trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. Only three top cross hybrid parents in Medani have negative GCA effects for plant height, i.e., L7, L3 and T3; they were best combiners for short plant type. Tallness which is an undesirable trait is shown by parents L1, L2 and T1 (Table 5). Crosses having negative SCA effects and consequently short plant type were T4 x L2, T1 x L4 and T2 x L4, while, tall hybrids with positive SCA effects were T3 x L1, T2 x L5 and T4 x L5 (Table 6).

The best combiners for the short plant type with negative GCA in Mutaq were L7, L6 and L2 while, the taller parents with high positive GCA effects were L5 and L1 (Table 5). Among the crosses the shortest hybrids were T2 x L4, followed by T3 x L5 and the tallest hybrids were T2 x L5 and T3 x L4 (Table 6). This showed that, there is a relationship between late flowering and tall plant type. This is quite obvious among the hybrids such as T3 x L1 and T3 x L4. Contribution for this trait is higher in crosses (80% and 53%) compared to parents (20% and 40%) at the two locations (Figs 1 and 2). The earliness and shortness are desirable traits especially under rainfed conditions for better water use efficiency and the escape of drought and avoidance of late season infestation with stem borer.

Ear length is a good index for higher grain yield, therefore any increase in ear length would be expected to increase number of kernels/row and hence directly improve grain yield. In Medani site, the long ear length parents having a positively significant GCA effects such as L5, L7 and T1, while parents showing the short ear length were L4 and L2 (Table 5). The best crosses for this trait having a positive SCA effects and hence the longest ear length were T2 x L5 and T4 x L7. On the other hand the best combiners in Mutaq were L7 and T3 (Table 5), while the best crosses were T1 x L1 and T4 x L4 (Table 6). In the two locations, the best contribution was (73% and 65.9%) obtained by SCA compared with (27% and44.1) for GCA (Figs1 and 2). These results emphasized that ear length has a direct effect for improving grain yield. This is in agreement with the finding of Vedia and Claure (1995) who found that ear aspect was the most important yield component.

Based on GCA estimates, the best combiners for ear diameter and length in Medani are L1 and L5, while best crosses were T1 x L2, T3 x L5 and T3 x L7. The good combiners in Mutaq site are L2, L3 and L4, while the best crosses are T3 x L4, T4 x L1 and T1 x L5 (Tables 5 and 6). A higher contribution among this trait is obtained by SCA (55.9% and 65.9%) in both locations compared with GCA (Figs 1and 2).

Favorable GCA values were given by T1 and L3 as the good combiners for kernel weight in Medani and the best crosses were T4 x L7 and T2 x L7. Among the studied parent material in Mutaq, only three parents have positive GCA effects (L3, L4 and T3).

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 1. Parent contribution of the maize GCA and SCA to the total variance

of yield and its componets at Medani, season 2008.

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 2. Parent contribution of the maize GCA and SCA to the total variance of yield and its componets at Mutaq, season 2008.

The best crosses were shown by T1 x L4 and T3 x L4 (Tables 5 and 6). The higher average contribution was given by the SCA (50.8% and 61) compared with the GCA at two locations (Figs 1and 2). This indicted that the inheritance of this trait was controlled by non additive gene effects.

At Medani site, all the results depicted in Table 5 showed that the parents differ considerably with respect to estimate of GCA effects for grain yield. The parents having positive GCA effects were T1 followed by L4 and L6. Parents having negative GCA effects were L2 and L6. The best crosses having positive SCA effects were T3 x L3 followed by T4 x L5 and T1 x L2. Negative SCA effects were shown by T3 x L4, T2 x L2 and T1 x L4 (Table 6). The higher combiner in Mutaq, were L2, L1 and L4. The best crosses were T3 x L5, T1 x L5 and T4 x L5, while negative SCA effects were shown by T1 x L3, T2 x L5 and T4 x L3 (Tables 5 and6). The great contribution was given by SCA (62.4% and 62%) compared with GCA at the two locations (Figs1 and 2).

General combing ability variance for grain yield is greater than the mean square for specific combining ability indicating the importance of additive gene action in controlling grain yield. This finding is in agreement with that of Barakat and Osman (2008) who found GCA effects are larger than SCA effects for grain yield indicating that the additive genetic variance is a major source of variations responsible for inheritance of grain yield.

CONCLUSION

The ratio of general combining ability variance for grain yield was greater than specific combining ability indicating the importance of additive gene action in controlling this trait hence the good combiner parent for grain yield across locations was L4 so it could be used in recurrent selection. Also enormous variability was detected in the studied population which makes cyclic selection more effective. The best cross was T4 x L5 indicating that dominance and epesitic interaction seemed to be predomint, hence, higher heterosis gratified and recommended cross T4 x L5 for future testing in multi-locations trials for commercial utilization in order to be released as a hybrid.

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.

Tallness is not a good character in grain maize production, since tall maize plants tend to be susceptible to stem and root lodging. Highly significant differences for tallness were detected among the studied parents with the general mean being of 144.4 cm. The trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. In the studied parents mean plant height ranged between 131.1 cm for L7 to 158 cm for T3 which was the tallest and latest parent across locations (Table 2). The crosses mean varied from 135.1 cm for (T3 x L7) to 155.9 cm for (T2 x L1).The tallest hybrids across locations were T4 x L6 and T4 x L3 (154 cm) (Table 3).

Table 3. Performance of 28 crosses for the measured traits in maize at the two locations combined, season 2008.

The results indicate that crosses were later than their parents. Also, the taller crosses were late maturing than short ones. Generally, the crosses were taller than their parents which suggested some degree of hybrid vigor.

Ear length trait is an important selection index for grain yield in maize. The ear length means of parents, as expected, were found to be shorter than those of the crosses at the two sites, with the general mean of 13.5 cm. The parents mean ranged between 12.7 cm for L7 to 15 cm for L2 (Table 2). The crosses mean varied from 11.9 cm for (T3 x L4) to 16.1 cm for (T3 x L5). However, long ear length were recorded for crosses T1 x L7 (15.2 cm), and T2 x L6 (14.6cm) (Table 3).Vedia and Claure (1995) found that ear length was the most important yield component and when used as a selection index genetic gain in recurrent selection reached 9.94% for yield and 5.75% for the ear traits. Therefore, any increase in ear length would be expected to increase number of kernels/row and hence increase grain yield.

Ear diameter is a good indicator of the number of kernel rows/ear. The mean of ear diameter across sites for parents ranged between 3.4 cm for L6 and L7 to 4.1 cm for L4 (Table 2). Among the crosses, the large ear diameter ranged from 3.3 cm for T3 x L6 to 4.0 cm for T2 x L1. The crosses which had a big ear diameter were T3 x L3 and T4 x L6 (3.8cm) (Table 3). This result was in agreement with the findings of Tracy (1990) who found that the maize hybrids with high yield had more ears/plant, longer ears and a better ear shape and row configuration.

The mean of one hundred kernels weight for parents was 21.4 g, and it ranged between 19.9 g for L2 to 22.8 g for T3 (Table 2). Among the crosses, the mean was 21.3 g. The best crosses which obtained the highest kernel weight were T1 x L2 and T4 x L3 (23.2) followed by T2 x L3 (22.8 g) (Table 3).

Yield is a polygenic character is influenced by the fluctuating enviro-nment. Moreover, it is a complex trait depending on many components (Sharaan and Ghallab, 1997). In this study, there was a considerable amount of variability among the genotypes for this trait. The studied parents in the two locations showed a general mean of 2.4 t/ha. The parents means ranged between 2.12 t/ha for L4 to 2.93 t/ha for T4 (Table 2), while, the crosses means ranged between 2.0 t/ha for (T2 x L5) to 3.55 t/ha for (T2 x L7) (Table 3). Most of the crosses (19 hybrids) had significantly higher mean grain yield than the overall mean. It is of interest to mention that the top ranking and the best yielder hybrids were T1 x L2 (3.4 t/ha), T2 x L1 (3.3 t/ha), T4 x L4 (3.3 t/ha), T1 x L1 (3.30 t/ha) and T4 x L3 (3.1 t/ha). These results agreed with those of Khalafalla and Abdalla (1997), who pointed to the fact that hybrids (crosses) produce higher grain yields than the open pollinated varieties due to the good performance of hybrids under Sudan conditions.

Combining ability

The breeding method to be adopted for improvement of a crop depends primarily on the nature of gene action involved in the expression of quantitative traits of economic importance. Combining ability leads to identification of parents with general combining ability effects and in locating cross combining showing high specific combining ability effects. In this study the ratio of GCA to SCA mean variance for most traits was less than one, suggesting that the inheritance of these traits was due to non additive gene action, with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and largely

This result indicates that dominance and epistatic interaction effects seemed to be predomint for this trait and therefore heterosis breeding may be gratifying. The good combiner parents, those having negative GCA effects in Medani, for 50% days to tasseling were L5 followed by T4, T1 and L7, indicating earliness for flowering time, while, the latest, having positive GCA effect was T3 (Table 5).The earliest crosses having negative SCA effects were T3 x L6, T2 x L7 and T2 x L4, while, the latest crosses were T2 x L5, T4 x L5 and T4 x L4 (Table.6).

The earliest parent in Mutaq was L7 (Table 5) and the earliest crosses were T2 x L4, T4 x L4 and T3 x L4 (Table 6). Common parents across locations that contributed to earliness were T4 and L5. The latest were L6 followed by T3 and T2 (Table 5). Parent L4 had good contribution for earliness to their hybrids progeny across locations.

Thus, the inbred lines which exhibited good general combining ability for at least one character can be used for development of early maturity and high grain yield. The contribution of the total variance for general and specific combining ability for this trait differs from location to another, but SCA was high in both locations (50.4% and 71.7%) compared with GCA which indicates that this trait is controlled by additive gene action (Figs 1 and 2).

Trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. Only three top cross hybrid parents in Medani have negative GCA effects for plant height, i.e., L7, L3 and T3; they were best combiners for short plant type. Tallness which is an undesirable trait is shown by parents L1, L2 and T1 (Table 5). Crosses having negative SCA effects and consequently short plant type were T4 x L2, T1 x L4 and T2 x L4, while, tall hybrids with positive SCA effects were T3 x L1, T2 x L5 and T4 x L5 (Table 6).

The best combiners for the short plant type with negative GCA in Mutaq were L7, L6 and L2 while, the taller parents with high positive GCA effects were L5 and L1 (Table 5). Among the crosses the shortest hybrids were T2 x L4, followed by T3 x L5 and the tallest hybrids were T2 x L5 and T3 x L4 (Table 6). This showed that, there is a relationship between late flowering and tall plant type. This is quite obvious among the hybrids such as T3 x L1 and T3 x L4. Contribution for this trait is higher in crosses (80% and 53%) compared to parents (20% and 40%) at the two locations (Figs 1 and 2). The earliness and shortness are desirable traits especially under rainfed conditions for better water use efficiency and the escape of drought and avoidance of late season infestation with stem borer.

Ear length is a good index for higher grain yield, therefore any increase in ear length would be expected to increase number of kernels/row and hence directly improve grain yield. In Medani site, the long ear length parents having a positively significant GCA effects such as L5, L7 and T1, while parents showing the short ear length were L4 and L2 (Table 5). The best crosses for this trait having a positive SCA effects and hence the longest ear length were T2 x L5 and T4 x L7. On the other hand the best combiners in Mutaq were L7 and T3 (Table 5), while the best crosses were T1 x L1 and T4 x L4 (Table 6). In the two locations, the best contribution was (73% and 65.9%) obtained by SCA compared with (27% and44.1) for GCA (Figs1 and 2). These results emphasized that ear length has a direct effect for improving grain yield. This is in agreement with the finding of Vedia and Claure (1995) who found that ear aspect was the most important yield component.

Based on GCA estimates, the best combiners for ear diameter and length in Medani are L1 and L5, while best crosses were T1 x L2, T3 x L5 and T3 x L7. The good combiners in Mutaq site are L2, L3 and L4, while the best crosses are T3 x L4, T4 x L1 and T1 x L5 (Tables 5 and 6). A higher contribution among this trait is obtained by SCA (55.9% and 65.9%) in both locations compared with GCA (Figs 1and 2).

Favorable GCA values were given by T1 and L3 as the good combiners for kernel weight in Medani and the best crosses were T4 x L7 and T2 x L7. Among the studied parent material in Mutaq, only three parents have positive GCA effects (L3, L4 and T3).

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 1. Parent contribution of the maize GCA and SCA to the total variance

of yield and its componets at Medani, season 2008.

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 2. Parent contribution of the maize GCA and SCA to the total variance of yield and its componets at Mutaq, season 2008.

The best crosses were shown by T1 x L4 and T3 x L4 (Tables 5 and 6). The higher average contribution was given by the SCA (50.8% and 61) compared with the GCA at two locations (Figs 1and 2). This indicted that the inheritance of this trait was controlled by non additive gene effects.

At Medani site, all the results depicted in Table 5 showed that the parents differ considerably with respect to estimate of GCA effects for grain yield. The parents having positive GCA effects were T1 followed by L4 and L6. Parents having negative GCA effects were L2 and L6. The best crosses having positive SCA effects were T3 x L3 followed by T4 x L5 and T1 x L2. Negative SCA effects were shown by T3 x L4, T2 x L2 and T1 x L4 (Table 6). The higher combiner in Mutaq, were L2, L1 and L4. The best crosses were T3 x L5, T1 x L5 and T4 x L5, while negative SCA effects were shown by T1 x L3, T2 x L5 and T4 x L3 (Tables 5 and6). The great contribution was given by SCA (62.4% and 62%) compared with GCA at the two locations (Figs1 and 2).

General combing ability variance for grain yield is greater than the mean square for specific combining ability indicating the importance of additive gene action in controlling grain yield. This finding is in agreement with that of Barakat and Osman (2008) who found GCA effects are larger than SCA effects for grain yield indicating that the additive genetic variance is a major source of variations responsible for inheritance of grain yield.

CONCLUSION

The ratio of general combining ability variance for grain yield was greater than specific combining ability indicating the importance of additive gene action in controlling this trait hence the good combiner parent for grain yield across locations was L4 so it could be used in recurrent selection. Also enormous variability was detected in the studied population which makes cyclic selection more effective. The best cross was T4 x L5 indicating that dominance and epesitic interaction seemed to be predomint, hence, higher heterosis gratified and recommended cross T4 x L5 for future testing in multi-locations trials for commercial utilization in order to be released as a hybrid.

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.

Tallness is not a good character in grain maize production, since tall maize plants tend to be susceptible to stem and root lodging. Highly significant differences for tallness were detected among the studied parents with the general mean being of 144.4 cm. The trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. In the studied parents mean plant height ranged between 131.1 cm for L7 to 158 cm for T3 which was the tallest and latest parent across locations (Table 2). The crosses mean varied from 135.1 cm for (T3 x L7) to 155.9 cm for (T2 x L1).The tallest hybrids across locations were T4 x L6 and T4 x L3 (154 cm) (Table 3).

Table 3. Performance of 28 crosses for the measured traits in maize at the two locations combined, season 2008.

The results indicate that crosses were later than their parents. Also, the taller crosses were late maturing than short ones. Generally, the crosses were taller than their parents which suggested some degree of hybrid vigor.

Ear length trait is an important selection index for grain yield in maize. The ear length means of parents, as expected, were found to be shorter than those of the crosses at the two sites, with the general mean of 13.5 cm. The parents mean ranged between 12.7 cm for L7 to 15 cm for L2 (Table 2). The crosses mean varied from 11.9 cm for (T3 x L4) to 16.1 cm for (T3 x L5). However, long ear length were recorded for crosses T1 x L7 (15.2 cm), and T2 x L6 (14.6cm) (Table 3).Vedia and Claure (1995) found that ear length was the most important yield component and when used as a selection index genetic gain in recurrent selection reached 9.94% for yield and 5.75% for the ear traits. Therefore, any increase in ear length would be expected to increase number of kernels/row and hence increase grain yield.

Ear diameter is a good indicator of the number of kernel rows/ear. The mean of ear diameter across sites for parents ranged between 3.4 cm for L6 and L7 to 4.1 cm for L4 (Table 2). Among the crosses, the large ear diameter ranged from 3.3 cm for T3 x L6 to 4.0 cm for T2 x L1. The crosses which had a big ear diameter were T3 x L3 and T4 x L6 (3.8cm) (Table 3). This result was in agreement with the findings of Tracy (1990) who found that the maize hybrids with high yield had more ears/plant, longer ears and a better ear shape and row configuration.

The mean of one hundred kernels weight for parents was 21.4 g, and it ranged between 19.9 g for L2 to 22.8 g for T3 (Table 2). Among the crosses, the mean was 21.3 g. The best crosses which obtained the highest kernel weight were T1 x L2 and T4 x L3 (23.2) followed by T2 x L3 (22.8 g) (Table 3).

Yield is a polygenic character is influenced by the fluctuating enviro-nment. Moreover, it is a complex trait depending on many components (Sharaan and Ghallab, 1997). In this study, there was a considerable amount of variability among the genotypes for this trait. The studied parents in the two locations showed a general mean of 2.4 t/ha. The parents means ranged between 2.12 t/ha for L4 to 2.93 t/ha for T4 (Table 2), while, the crosses means ranged between 2.0 t/ha for (T2 x L5) to 3.55 t/ha for (T2 x L7) (Table 3). Most of the crosses (19 hybrids) had significantly higher mean grain yield than the overall mean. It is of interest to mention that the top ranking and the best yielder hybrids were T1 x L2 (3.4 t/ha), T2 x L1 (3.3 t/ha), T4 x L4 (3.3 t/ha), T1 x L1 (3.30 t/ha) and T4 x L3 (3.1 t/ha). These results agreed with those of Khalafalla and Abdalla (1997), who pointed to the fact that hybrids (crosses) produce higher grain yields than the open pollinated varieties due to the good performance of hybrids under Sudan conditions.

Combining ability

The breeding method to be adopted for improvement of a crop depends primarily on the nature of gene action involved in the expression of quantitative traits of economic importance. Combining ability leads to identification of parents with general combining ability effects and in locating cross combining showing high specific combining ability effects. In this study the ratio of GCA to SCA mean variance for most traits was less than one, suggesting that the inheritance of these traits was due to non additive gene action, with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and largely

This result indicates that dominance and epistatic interaction effects seemed to be predomint for this trait and therefore heterosis breeding may be gratifying. The good combiner parents, those having negative GCA effects in Medani, for 50% days to tasseling were L5 followed by T4, T1 and L7, indicating earliness for flowering time, while, the latest, having positive GCA effect was T3 (Table 5).The earliest crosses having negative SCA effects were T3 x L6, T2 x L7 and T2 x L4, while, the latest crosses were T2 x L5, T4 x L5 and T4 x L4 (Table.6).

The earliest parent in Mutaq was L7 (Table 5) and the earliest crosses were T2 x L4, T4 x L4 and T3 x L4 (Table 6). Common parents across locations that contributed to earliness were T4 and L5. The latest were L6 followed by T3 and T2 (Table 5). Parent L4 had good contribution for earliness to their hybrids progeny across locations.

Thus, the inbred lines which exhibited good general combining ability for at least one character can be used for development of early maturity and high grain yield. The contribution of the total variance for general and specific combining ability for this trait differs from location to another, but SCA was high in both locations (50.4% and 71.7%) compared with GCA which indicates that this trait is controlled by additive gene action (Figs 1 and 2).

Trends in breeding work are to develop cultivars that are dwarf or of moderate height to avoid lodging of the crop which adversely affects yield. Only three top cross hybrid parents in Medani have negative GCA effects for plant height, i.e., L7, L3 and T3; they were best combiners for short plant type. Tallness which is an undesirable trait is shown by parents L1, L2 and T1 (Table 5). Crosses having negative SCA effects and consequently short plant type were T4 x L2, T1 x L4 and T2 x L4, while, tall hybrids with positive SCA effects were T3 x L1, T2 x L5 and T4 x L5 (Table 6).

The best combiners for the short plant type with negative GCA in Mutaq were L7, L6 and L2 while, the taller parents with high positive GCA effects were L5 and L1 (Table 5). Among the crosses the shortest hybrids were T2 x L4, followed by T3 x L5 and the tallest hybrids were T2 x L5 and T3 x L4 (Table 6). This showed that, there is a relationship between late flowering and tall plant type. This is quite obvious among the hybrids such as T3 x L1 and T3 x L4. Contribution for this trait is higher in crosses (80% and 53%) compared to parents (20% and 40%) at the two locations (Figs 1 and 2). The earliness and shortness are desirable traits especially under rainfed conditions for better water use efficiency and the escape of drought and avoidance of late season infestation with stem borer.

Ear length is a good index for higher grain yield, therefore any increase in ear length would be expected to increase number of kernels/row and hence directly improve grain yield. In Medani site, the long ear length parents having a positively significant GCA effects such as L5, L7 and T1, while parents showing the short ear length were L4 and L2 (Table 5). The best crosses for this trait having a positive SCA effects and hence the longest ear length were T2 x L5 and T4 x L7. On the other hand the best combiners in Mutaq were L7 and T3 (Table 5), while the best crosses were T1 x L1 and T4 x L4 (Table 6). In the two locations, the best contribution was (73% and 65.9%) obtained by SCA compared with (27% and44.1) for GCA (Figs1 and 2). These results emphasized that ear length has a direct effect for improving grain yield. This is in agreement with the finding of Vedia and Claure (1995) who found that ear aspect was the most important yield component.

Based on GCA estimates, the best combiners for ear diameter and length in Medani are L1 and L5, while best crosses were T1 x L2, T3 x L5 and T3 x L7. The good combiners in Mutaq site are L2, L3 and L4, while the best crosses are T3 x L4, T4 x L1 and T1 x L5 (Tables 5 and 6). A higher contribution among this trait is obtained by SCA (55.9% and 65.9%) in both locations compared with GCA (Figs 1and 2).

Favorable GCA values were given by T1 and L3 as the good combiners for kernel weight in Medani and the best crosses were T4 x L7 and T2 x L7. Among the studied parent material in Mutaq, only three parents have positive GCA effects (L3, L4 and T3).

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 1. Parent contribution of the maize GCA and SCA to the total variance

of yield and its componets at Medani, season 2008.

DT= days to 50% tasseling, PH= plant height (cm), EL= ear length (cm)

ED= ear diameter (cm), KW= kernels weight (g), GY= grain yield (t/ha)

Figure 2. Parent contribution of the maize GCA and SCA to the total variance of yield and its componets at Mutaq, season 2008.

The best crosses were shown by T1 x L4 and T3 x L4 (Tables 5 and 6). The higher average contribution was given by the SCA (50.8% and 61) compared with the GCA at two locations (Figs 1and 2). This indicted that the inheritance of this trait was controlled by non additive gene effects.

At Medani site, all the results depicted in Table 5 showed that the parents differ considerably with respect to estimate of GCA effects for grain yield. The parents having positive GCA effects were T1 followed by L4 and L6. Parents having negative GCA effects were L2 and L6. The best crosses having positive SCA effects were T3 x L3 followed by T4 x L5 and T1 x L2. Negative SCA effects were shown by T3 x L4, T2 x L2 and T1 x L4 (Table 6). The higher combiner in Mutaq, were L2, L1 and L4. The best crosses were T3 x L5, T1 x L5 and T4 x L5, while negative SCA effects were shown by T1 x L3, T2 x L5 and T4 x L3 (Tables 5 and6). The great contribution was given by SCA (62.4% and 62%) compared with GCA at the two locations (Figs1 and 2).

General combing ability variance for grain yield is greater than the mean square for specific combining ability indicating the importance of additive gene action in controlling grain yield. This finding is in agreement with that of Barakat and Osman (2008) who found GCA effects are larger than SCA effects for grain yield indicating that the additive genetic variance is a major source of variations responsible for inheritance of grain yield.

CONCLUSION

The ratio of general combining ability variance for grain yield was greater than specific combining ability indicating the importance of additive gene action in controlling this trait hence the good combiner parent for grain yield across locations was L4 so it could be used in recurrent selection. Also enormous variability was detected in the studied population which makes cyclic selection more effective. The best cross was T4 x L5 indicating that dominance and epesitic interaction seemed to be predomint, hence, higher heterosis gratified and recommended cross T4 x L5 for future testing in multi-locations trials for commercial utilization in order to be released as a hybrid.

The development of hybrids is the main objective of maize breeding. However, success depends largely on the identification of the best parents to ensure maximum combining ability. This study was conducted to estimate genetic variability and combining ability for grain yield and yield components of seven local inbred lines and four introduced open pollinated varieties of maize (Zea mays L.) across two irrigated locations, Medani and Matuq, Gezira, Sudan in 2008. The experiment was arranged in a randomized complete block design with three replicates. The traits measured were days to 50% tassel, plant height, ear length, ear diameter, hundred kernels weight and grain yield. Significant differences were observed among the parents and crosses for most of studied traits in both seasons. The crosses showed high genetic variability and tall plants than their parents which suggested some degree of hybrid vigor. The tallest hybrids across locations were T3 x L5 and T4 x L3. This indicates that the crosses were late maturing than their parents. The highest yielding hybrids had long ears and better shape, e.g., T2 x L1 and T1 x L7.The top five ranking crosses for grain yield across locations were T2 x L7 (3.45 t/ha), T1 x L2 (3.44 t/ha), T2 x LI (3.32 t/ha), T4 x L4 (3.30 t/ha) and T1 x L1 (3.13 t/ha). The inheritance of most traits was controlled by non-additive gene action except ear height and grain yield. The best combiners for grain in Medani were T4, L4 and L5, while in Mutaq were L2, L4 and L6. The ratio of GCA to SCA variance for the most traits was less than one, suggesting that the inheritance was due to non additive gene effect with the exception of grain yield being more than one, indicating that inheritance of this trait was due to GCA effects, and was largely controlled by additive gene action in the base material. From these results it is recommended that parents T4, L1 and L6 to be used in recurrent selection, while, crosses T3 x L5, T1 x L5 and T4 x L6 to be tested in multi-locations trials for commercial utilization.

INTRODUCTION

Maize generally is one of the most diverse crop both genetically and phenotypically. Due to its wide adaptability and productivity, maize spread rapidly around the world after the Europeans brought the crop from the Americas in the 15th and 16th centuries (McCann, 2005). The Portuguese introduced the crop to Africa at the beginning of the 16th century and since then the crop has replaced sorghum and millet as the main staple food in most of the continent where the climatic conditions are favorable (McCann, 2005). Today, there is an increasing interest in maize production in Sudan due to its suitability to cultivation in the agricultural irrigated schemes, especially in the Gezira.It can occupy an important position in the economy of the country due to the possibility of blending it with wheat for making bread (Nour et al., 1997; Meseka, 2000).

The grain yield of existing maize varieties and local landraces in Sudan is low. Also, maize hybrids have been reported to show high potential for grain yield than the open pollinated varieties and landraces (Alhussein, 2007). Advantages of hybrids over open pollinated cultivars are higher yield, uniformity, high quality and resistance to diseases and pests. In spite of having yield potential, the production of maize in Sudan is very low. One of the reasons for this is the cultivation of exotic hybrids, which are not well adapted to our agro-climatic conditions. One of the strategies of the Agricultural Research Corporation (ARC) of the Sudan for maize breeding program is to develop new hybrids as an attempt to incorporate both advantages for higher yield and adaptability to environmental conditions. Thus, getting the benefit from the use of hybrids is the main purpose in maize breeding program of ARC. Therefore, the objective of this study is to estimate the magnitude of combining ability in 28 topcross hybrids of maize for grain yield and its components across two irrigated locations and to identify high yielding topcross hybrids for future testing and commercial utilization.

MATERIALS AND METHODS

The plant material used consisted of 7 local inbred lines used as lines (L), and 4 introduced open pollinated varieties used as testers (T) crossed in line x tester arrangement (Table 1). Hand pollination was used to develop the breeding material. Pollen grain was collected into a paper bag from the tassel of male parent (tester) and then dusted on the silk of the female parent (line). The ear was covered with a bag and information regarding the cross was written on the bag. A total of 28 cross combinations was obtained through hand pollination. In July 2008, the 11 parental material and 28 cross hybrids were grown and evaluated at two irrigated locations, Medani, Gezira Research Station (GRS) and Matuq, Matuq Research Station (MRS), Gezira State, Sudan. The trials were arranged a randomized complete block design with three replicates. The plot size was maintained as 2 rows x 3 m long with inter and intra row spacings of 80 and 25 cm, respectively. Seeds were sown at the rate of 3- 4 seeds per hill. Plants were thinned to one plant per hill after three weeks from sowing. Nitrogen was applied at 86 kg/ha in a split dose after thinning and before flowering. The crop was irrigated at intervals of 10-14 days, and plots were kept free of weeds by hand weeding. Data were analyzed using the Statistical Analysis System (SAS) computer package. The analysis was done for each season for characters days to 50% tasseling, plant height, ear length, ear diameter, kernels weight and grain yield and then combined. Mean performance was separated using Duncan's Multiple Range Test (DMRT). Data from each location was analyzed separately and across locations to determine the general and specific combining ability of each line was measured according to Griffing,s Method 2 (1956).

Table 1. Pedigree of the lines and testers used in the study.

Parents

Pedigree

Source

L1

RING-B-S1-2

Inbred line developed by ARC

L2

PR-89 B-5655-S1-1

Inbred line introduced from CIMMYT, Mexico

L3

RING-B- S1-3

Inbred line developed by ARC

L4

RING- B-S1-1

Inbred line developed by ARC

L5

RING-A-S1-1

Inbred line developed by ARC

L6

RING-A-S1-2

Inbred line developed by ARC

L7

PR-89 B-5655-S1-3

Inbred line introduced from CIMMYT, Mexico

T1

SOBSIY-HG AB

OPV introduced from CIMMYT, Kenya

T2

ACROSS- 500 HGY-B

OPV introduced from CIMMYT, Kenya

T3

CORRALE10 -02 SIYQ

OPV introduced from CIMMYT, Kenya

T4

BAILO- 02SIYQ

OPV introduced from CIMMYT, Kenya

RESULTS AND DISCUSSION

The performance of the material tested for most traits is high across the two locations. However, significant differences among the parents and their hybrids for most traits were shown indicating the diversity of the material.

Mean separation and ranking

Mean days to 50% tasseling indicates that the pollen shedding ability of maize genotypes is an indicator of the earliness of genotypes. Mean days to tasseling across locations for parents scored 52 days as the general mean. Mean of parents ranged between 49 and 55 days for L6 and T3, respectively (Table 2). The mean of crosses ranged between 46 days for (T4 x L5) to 52 days for (T2 x L1) (Table 3). Identification of early tasseling genotypes is very important in developing hybrids and choosing hybrids to suit different agro-ecological zones as well as grower’s requirements. Earliness was a desirable trait especially under rainfed conditions. It is important for better use of water resources and avoidance of late season infestation with stem borers. Hence, the earliest crosses were T1 x L7 (47 days), T4 x L7 (47 days), T4 x L4 (48 days) and T4 x L6 (48 days) (Table 3).

Table 2. Mean performance of eleven parents for the measured traits in maize at the two locations, season 2008.